1. Field of the Invention
The present invention relates to a thermal treatment system for semiconductors, which is used for depositing a non-oxide film, such as a polysilicon film or a nitride film, or an oxide film on a surface of a semiconductor wafer.
2. Description of the Related Art
For the outer tube or the inner tube used in low pressure CVD systems for thermal treatment of semiconductors and reactors for high-temperature thermal treatment, quartz glass has been used, for example, for reasons that it is easy to obtain high-purity glass, it has a thermal resistance, it has a small thermal stress generated due to a low thermal expansion coefficient, and it is excellent at thermal insulation due to a low thermal conductivity. When a deposited film is a polysilicon film or a nitride film, a thermal treatment system for semiconductors, which uses, e.g., an outer tube made of silicon carbide, has been recently proposed from the viewpoints that the difference in thermal expansion coefficient between the deposited film and quartz glass causes the deposited film on the quartz glass to peel off in the system and contaminate a wafer and that the thermal resistance is further improved (see Patent Documents 1 and 2).
However, the use of silicon carbide has a problem that fracture is easily caused due to tensile stresses or bending stresses mainly generated at three locations A, B and C shown in FIG. 7 of Patent Document 2 (corresponding to FIG. 4 of the application) since silicon-carbide has a higher thermal expansion coefficient and a higher thermal conductivity than quartz glass. The use of silicon carbide also has a problem that an O-ring, which is normally interposed between the outer tube and a base, is apt to be seized because of silicon carbide having a high thermal conductivity, and the gas-sealing ability is therefore impaired easily.
As one of the measures, it has been proposed a method (hereinbelow, referred to as measure A) wherein the distance between the bottom surface of an outer tube made of silicon carbide and the lowest end of a heater is set at a length of 200 mm or longer in order to locate an O-ring physically away from a heat source (see Patent Document 1). As another measure, it has been proposed a method (hereinbelow, referred to as measure B) wherein a seal ring is interposed between a flange of an outer tube made of silicon carbide and a base and wherein an inner peripheral portion of the flange that is radially more inward than the seal ring is contacted to and supported by the base (see Patent Document 2).
However, it has been recently difficult to ensure a length of 200 mm or longer. This is because it is strongly demanded to deal with a large volume of silicon wafers at a time and because there is a tendency to spread an isothermal heating zone or to bring the lower end of a heater as near to a base as possible in order to increase the number of silicon wafers to be dealt with in a thermal treatment system for semiconductors, such as a low pressure CVD system. From this viewpoint, measures other than measure A have been demanded more and more.
Additionally, the diameter of silicon wafers has been further increased from 200 mm to 300 mm or longer, and the outer diameter of outer tubes has been accordingly increased to 350 mm or longer. For this reason, in the case of adopting measure B, there is a possibility that the flange of an outer tube is insufficiently cooled. There is also a possibility that the load pressure applied to a sealing member varies according to temperature to cause gas leakage due to an insufficient sealing pressure in particular at a time of carrying out treatment at a low temperature since an inner peripheral portion of the flange supported by the base has line contact and since the contact location changes according to a thermal treatment temperature.
In other words, there have been proposed no measures other than measure A or measure B, which can satisfy the requirements, such as an increase in diameter, an increase in, throughput, and prevention of contamination caused by particles, which can minimize the limitations in the shape and the way of use of an outer tube or the like to be used, and which can make it difficult for the outer tube or the like to be fractured by a thermal stress, have sufficient durability and be excellent at sealing ability without supporting the outer tube or the like at an inner peripheral portion.
Patent Document 1
JP-A-9-251991 (page 1 to page 7 and FIG. 1)
Patent Document 2
JP-A-10-195657 (page 1 to page 8 and FIGS. 1 to 7, in particular FIG. 7)
Patent Documents 1 and 2 correspond to U.S. Pat. No. 5,902,406 and EP-A-0795897.